Screw fixation in lag fashion of equine cadaveric metacarpal and metatarsal condylar bone specimens: a biomechanical comparison of shaft and cortex screws.

This is a biomechanical research study that investigates the stability and insertion efforts of different screw types in equine bone. The results suggest that cortex screws with a long shaft offer better stability and require less insertion effort than screws with a short shaft or no shaft.

Research Design

• The focus of this study is to compare the stability and insertion effort between different cortex bone screw types when inserted into equine metacarpal (MC(T)III) bone.
• This study utilized bone specimens taken from the condyles of the distal equine MC(T)III.
• Different screw types were examined with variable elements such as screw diameter (either 4.5 or 5.5mm) and the presence or absence of a shaft (either without a shaft, with a 20-mm shaft, or with a 25-mm shaft).
• After the insertion of screws into bone blocks, shear tests were conducted in a mechanical testing machine to assess various factors such as peak insertion torque, insertion energy, stiffness, yield strength, and displacement at 3kN load.

Results

• The results indicate considerable differences between screw designs. Screws that included a shaft were found to be 30%-40% stiffer and 60%-70% stronger than those without a shaft.
• The screws with a shaft could also endure 80 to 95kg more force before yielding as compared to screws without a shaft.
• In the instance of a 3kN load, the displacement with screws that had a shaft was only 55%-60% of the displacement experienced with screws without a shaft.
• The long shaft screws generally outperformed the ones with a short shaft. However, there was no significant difference in shear stiffness, shear yield strength, or shear displacement between the screw diameters of 4.5mm and 5.5mm.

Conclusion

• Despite the larger diameter screws requiring more effort to insert, and screws with a short shaft requiring more insertion energy, these differences were considered minor.
• In conclusion, the research suggests that cortex screws with a long 4.5- or 5.5-mm shaft offer better stability in equine MC(T)III condyle bone with less insertion effort compared with those with a short shaft or no shaft.
• Therefore, for the fixation of equine MC(T)III condylar fractures, the study suggests considering the usage of cortex bone screws inserted in a lag fashion with a shaft.